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CHAPTER 1: INTRODUCTION
Figure 1.10: Feedthrough capacitor with its symbol
To realize these filters, choke coils are used as inductors, with high frequency ferrite
cores commonly made out of iron powder. For the CM inductor all the phases are
wound on the same core in a way that the total flux created will be nearly zero. This
will allow the core to be designed only for a small fraction of the rated current, reducing
its size without compromising its effectiveness.
As regarding to the capacitors, they must have a low parasitic series inductance to
perform well at the high frequency involved. The bests capacitors under this aspect are
the feedthrough capacitors, Fig. 1.10. They have three contacts: two are the two sides
of a metal rod where the phase current flows, while all around it there is the dielectric
and an enclosure for the ground contact. They provide the shortest possible path for
the current to be coupled to the third contact (ground), granting the lowest equivalent
series inductance. These capacitors, nevertheless, are very application specific so are
both expensive and difficult to find. More common type of capacitors used are the
class X and Y capacitor. Their dielectric can be paper, metallized paper, polystyrene
or ceramic. The class specify the grade of electric insulation the capacitor is granted
to withstand, and it’s related to the application it is supposed to be used. X capacitors
are meant to be used across line phases, so that a short circuit will not cause any risk
to personnel. Y capacitor are supposed to be used as capacitance towards ground.
The use of these requires more strict rules to be followed because a short failure will
connect the phase directly to the ground (usually the casing), posing a serious threat of
electrical shock to the user.
1.5 Literature review and background
In the literature there are plenty of studies on the EMI problems related to power con-
verters. For example [13–18] to cite just a few, deal with HF interference mitigation for
normal PWM converters, suggesting solutions to attenuate the high-frequency noise
created by the switches by means of the introduction of passive L or LC EMI filters.
Others articles implement an active circuit to minimize these currents, such as [19, 20]
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